Helical Steel Stairs

[spats]

I know this issue has come up before, and I've read the old posts, but I have not seen a definitive answer on how to design helical steel stair stringers. I found reference to an out-of-print book "Staircase: Structural Analysis and Design" by M.Y.H. Bangash that sells for $166-$255, but I really don't want to spend that kind of money, and I don't know if it addresses steel members. Bangash's book refers to Morgan's Method and Cohen's Method, but I'm unable to find any literature for either of those references, and I'm not even sure they relate to steel stairs either.

An eng-tips post from 2000 refers to "Design Charts for helical stairs with fixed supports by A.R. Cusens & S. Santathadaporn", but I can't find that literature either.

I know there are big time torsional issues as well as thrust at the bottom of the stairs. Does anybody have a clue as where to start?

 

[JAE]

See attached - looks complex.

 

[racookpe1978]

Get a pre-fabricated steel assembly from one of the shops. You'll get fabricated steps, mounts, center post, handrails, decorative iron, handrail brackets, and step=off pieces delivered for less than 2000.00 - 3000.00.

Following from Google:

Shop for spiral staircase kits on Google
Steel Spiral Stair Kit Salte...
$987.00
Salter Spiral...
Stair KitsPhoenix...
$3099.00
Home Depot
60" Irc Code Steel Spiral S...
$1730.40
eBay
Spiral Staircase Kit...
$2095.00
Etsy
Stair KitsEureka S...
$2999.00
Home Depot

 

[spats]

Thanks JAE... the paper appears to be addressing a helical slab and not a helical stringer, but it has some good references at the end of the article that may lead somewhere.

racookpe... if only it had a center post and was that simple. This is a monumental "Gone With The Wind"-type stair.

 

[hokie66]

You need a miracle.

http://www.snopes.com/horrors/ghosts/loretto.asp

 

[racookpe1978]

Oh - One of "those" eh?

I've built a couple like that for high-end homes, but not for commercial. But in most, the swept, spiral curving wood steps and framing were already in place, and all I needed to do was design and style and bend and weld the wrought iron, design and bend the handrails and brackets, weld up and grind smooth everything, and fab the whole thing in place in the housewife's living room ...

Treat it like two springs being compressed: the outer spring steel is the outside frame, the inner spring (different radius and spiral) is the second. Don't get too fancy with the steps - they are a simple beam held at both ends. You want each "spring" to hold half your total steps load (nbr of steps x expected load per step / 2) without compressing too much - both should ideally compress the same actually. From compressed spring theory, then you "translate" the spring section modulus to a equivalent beam.

Then you roll and twist the two beams so they are equal distances apart. It is easier to roll and twist tube steel or solid bars, NOT I-beams/angles or channels or w shapes. The absolute easiest - because you HAVE to "twist" the beam sections as you turn them into a spiral is a solid round. The round bar is more forgiving as you twist it, because your eye won't see small changes and distortions. A flat bar must be twisted "perfectly" to look right.

 

[JStephen]

Maybe segment it into short sections and use canned analysis software?

 

[spats]

Thanks for the replies. Neat stairs and story hokie66. Good perspective racookpe, especially about using a tube section, but the problem is in modeling the spring. Segmenting in a plane is possible, but that ignores the 3D dimensionality of the stringer. Would modeling it in plan be conservative? It seems like it would be from a deflection and rotation standpoint, but how significant is the thrust at the bottom. Would all the vertical load essentially wind up at the bottom? Would the torsion be resisted somewhat equally at the top & bottom of the stringer? I'm connecting to columns at the top, with the stringer's torsion translating into bending moment in the columns, and requiring that the columns be properly tied laterally into the floor system.

 

[JStephen]

I meant a 3-D model with the helix modeled as a series of segmented sloped beams.

 

[racookpe1978]

Vertical load (static plus the load on each step) is essentially all carried through directly to the bottom as a net vertical load that must be resisted by the floor over the actual area of the steel frame that is touching the floor.

Sideways forces that are developed (created) are from the max distance (lever arm) between the points of attachment at the top (and the bottom) and the load as it walks down the spiral. This lever arm will act to "pull" the stair away from the upper platform

shear force at the bottom is small: think of a the sheer force caused by a screw thread: the sideways force from a screw thread is the product of the slope of the screw thread.